This invention relates to a process for imparting a permanent identification mark to an optically clear plastic article such as a lens. More particularly, it relates to a method of marking which enables a manufacturer to confirm that a returned article which had been distributed to the consumer is in fact its own product, yet results in a mark which has no material effect on the passage of visible light through the product.
Ophthalmic quality plastic lenses made of allyl diglycol carbonate (CR-39), polycarbonate (Lexan), and the like are now widely available. These products are distributed to the consumer through retail outlets which frequently sell products of more than one manufacturer. When a defective or damaged lens which has been mounted in an eyeglass frame is returned, it is often impossible to determine the identity of the lens' manufacturer.
An apparent way to remedy this difficulty is to provide an integral indicia of some kind or otherwise build within the lens structure a unique and identifiable characteristic. A satisfactory method of marking products of this type has not yet been found. Since ophthalmic quality lenses must be optically clear and cosmetically perfect they cannot be provided with a surface embossed design or colored mark which is permanently on or in the lens. Such a mark would likely be viewed as an imperfection or a flaw in the optical quality of the lens. Were the mark so small as to be clearly free of these disadvantages, it would likely be too small to be of practical value.
In most cases, the manufacturer distributes the products to retailers, such as optical laboratories, in the form of lens blanks which thereafter cut the blank to a lens size and subsequently glaze the lens into a frame. Because of this distribution procedure and because of the expense involved, it is not practical to affix an identifying mark on an edge of the lens which would be hidden by the eyeglass frame. Furthermore, because the mark must be permanent, the simple expedient of affixing a removable label on the lens is inappropriate.
The ideal identification mark for an optically clear lens should leave no flaw or blemish of any kind on the lens surface, should be permanent, should not be visually detectable, and should be easily applied at the factory site on both lenses and lens blanks.
With the above thoughts in mind one might be invited to conclude that the indicated solution of the present problem would be an invisible mark since such marks are widely known in the securities and banking field. Flourescent inks as applied to various documents such as bills and notes might be argued to suggest a solution for the present problem by stamping the ophthalmic lens with a fluorescent ink. Unfortunately the solution of the problem is not so straight forward as might be suggested. Plastic articles are not absorbent to the degree that papers and fabrics are. Further, any ink mark applied to the article would be very readily wiped clean by the normal action of cleaning ones eyeglass lens.
It is further desirable that the mark placed upon the plastic article (ophthalmic lens) be in a relatively small form such that inadvertent observation might not be a major distraction to the wearer or those about him. It is necessary therefor that the identifying mark be placed upon the article in rather small fashion and be of fairly high resolution. It has been found that inks and other liquids have a profound tendency to run on the surface of a plastic article, spreading in all directions with little controllability. For these and the above reasons, fluorescent inks such as known for use in the securities trade are not acceptable for solution to the present problem.
So the problem faced in the present invention then demands marking by other than a conventional liquid. Again those familiar with the fluorescent inks might be led to conclude that, since some inks are in fact fluorescent powders in solution, one might use powder to place an indication on the lens. This approach generates a problem of adhesion to the lens since the powders do not readily adhere. Should the powder be desolved in the ink liquid, the situation reverts to a marking liquid being placed upon the surface of the plastic article.
The present problems are solved in the invention by the method of marking the article with a fluorescent powder preferably in combination with powder diluent. These powders or solids are initially contained in a carrier liquid, perferably in solution, however suspensions are acceptable. The powders are then applied to the lens with the mixture (dye and carrier) in a super-saturated condition, such that the characteristics of the marking composition are not truly that of either a liquid or a powder. The composition might be compared to a paste or slurry.
In addition to the problems of application of the suitable dye for marking the plastic article there is also a problem of adequately applying the proper amount of dye such that a readily recognizable, high resolution small mark can be placed upon the article, e.g. one having a predetermined limited degree of ultraviolet response when activated. It is therefore necessary to meter the amount of dye being used during the marking process.
Characteristically, fluids are metered by the relative concentrations of components in a solution or suspension. In the present invention, however, the marking composition is not characteristically a liquid. A further problem in the present invention is that the concentrations of the marking solution are continually changing due to the necessity of having to mark the article with a super-saturated solution or suspension. We have found that by causing the carrier solution to volatilize such that the super-saturated state of the marking composition can be reached. This volatilization necessarily means that relative concentration of the ingredients including the dye continue to increase in concentration unless extraordinary measures are taken. The concentration of dye may thus reach a level which produces a mark which is over-responsive and thus unsatisfactory.
With this in mind, we use a very small quantity of dye material and perferably dilute the dye in a second powder. Peculiarly however, the characteristics of the powder must be markedly different than the dye so as to not interfere with the process by which the dye forms the permanent mark on the plastic article. We thus utilize a powder diluent which during the process of transferring a dye material to the face of the plastic article sublimes into the surrounding atmosphere so as to leave no trace of the diluent material on the face of the marked plastic article.